1. Field of the Invention
The present invention relates to a wire electric discharge machining apparatus having a function of determining the tendency of a wire cutting state during an automatic wire connecting operation to automatically select a cutting condition.
2. Description of the Related Art
In the case of occurrence of wire breakage or cutting for the next machining-rewind-tip cutting-wire connecting operation-failure, automatic wire connection in wire electric discharge machining is completed by processes of retry repetition-wire connecting through upper and lower die guides-detection of wire connection by detection part.
The automatic wire connection has two steps. The cutting of the tip of a wire electrode line as one of the steps is performed in order to eliminate the bending of the tip of the wire electrode line caused by the wire breakage during machining of a workpiece or remove a flaw on the surface of the wire electrode line that tends to be caught on upper and lower die guides that holds the wire electrode line above and below the workpiece to reproduce a fine tip required for smooth wire connection of the wire electrode line (i.e., a straight tip having a smooth surface).
The wire cutting operation is normally performed by annealing the wire electrode line. In this method, the wire electrode line to be annealed is clamped by a pair of current-carrying electrodes disposed in upper and lower portions of an automatic wire-connection unit, and the clamped wire electrode line is energized while an annealing torque is applied thereto. Subsequently, the wire electrode line annealed by electrical resistance heat generation is cut by generating a cutting start torque in a wire rewind direction using a brake roller. Cutting air injected into an upper pipe at the same time has an important role of determining the cutting position of the wire electrode line and stabilizing the straightness of the cut wire electrode line. Japanese Patent Application Laid-open No. 6-304819 discloses a technology in which an annealing effect when energization cutting is performed can be effectively utilized in a wire electric discharge machining apparatus having a wire cutting mechanism using an energization cutting method.
The wire electrode line of which the tip portion is already cut sequentially reaches, according to a predetermined sequence, upper die guide—workpiece—lower die guide—lower die guide roller—lower pipe—feed part (pinch roller, feed roller)—wire connection completion detection plate, whereby the automatic wire connection of the wire electrode line is completed.
At the same time, in the automatic wire connecting operation, a wire connection state and a wire connection tendency are constantly monitored by a retry position counter function indicative of the position of a wire connection failure of a retry and the number of wire connection failures. Accordingly, when the automatic wire connection has a problem and the retry frequently occurs at a specific position, the wire connection error factor peculiar to the position is pre-identified through experiences or experiments, and hence it is possible to track down the cause. Japanese Patent Application Laid-open No. H7-112328 discloses a technology in which the degree of an abnormal state of a position of each mechanism part of a wire electric discharge machining apparatus through which a wire electrode line passes is displayed in association with characteristics of clogging with the wire electrode line peculiar to the position. In addition, Japanese Patent Application Laid-open No. 7-24643 discloses a technology regarding wire trouble occurrence position detection in a wire electric discharge machining apparatus.
The sequence of the automatic wire connection is operated according to a series of preset parameters, and hence the sequence thereof is based on the assumption of a stable wire connection environment by optimum mechanical adjustment and quality of the wire electrode and the margin of an excellent cutting region is relatively small. Consequently, when a problem exceeding the margin occurs and it is not possible to cope with the problem using a default parameter set as the initial value, there has been a possibility that a cutting/wire connection error occurs and the problem eventually leads to an emergency suspension.
In addition, with the function of the automatic wire connection of the current electric discharge machining apparatus, it is possible to quickly narrow down the cause by monitoring of the retry position counter function, but the final correction is based on cutting/wire connection parameter correction manually performed by an operator. Consequently, the response to the factor for the cutting/wire connection error of the wire electrode line is varied depending on differences in experience and knowledge at the operator level, and there is a problem in terms of working efficiency. Further, in terms of the promotion of automation of machining of the workpiece, the retry position counter function is limited only to the monitoring function, and the retry position counter function has not been related directly to active correction of the error basically.